The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In the field of image sensors or optical sensors, a variety of techniques using a sensor capable of sensing photons having an infrared, visible or wide spectrum have been developed. However, none of these techniques are used in the mass production of imager devices with a three-dimensional multi-layer structure.
The reason for this is that each technique falls short in production use for layered sensors, is: 1) a reduction in fill factor due to the existence of signal processing and transfer circuits taking up space in the pixel; 2) that light having a variety of wavelengths is not sensed at a single pixel location, 3) optimization of a wavelength selection filter that is integrated into a device and can select specific spectra only all at one pixel location, 4) leakage current reduction; and finally 4) kTC noise reduction.
A sensor having three or more layered photodiodes can convert the whole of the incident light into image information like photographic film. A previous report by Wolfenbuttel showed the existence of such a sensor in which depth of absorption is different depending on wavelength in a silicon layer and thus depth could be used to classify the color of the incoming light. Furthermore, in other reports, a selective epitaxial growth method was used. In still another reports, an effort was made to obtain the same effect by controlling the depletion depth of amorphous silicon by changing operating bias and thereby controlling the depth from which photo generated charge is collected.
All the techniques have not been successfully commercialized as stated in their reports. A company who has developed such concept into mass-production is Foveon Inc. Foveon used a three-layered photodiode structure with collection at various depths as per the Wolfenbuttel scheme. However, Foveon's method has three problems in obtaining the above stated advantages.
First, a laminated diode structure cannot be formed with a “pinned” structure. Therefore, a problem arises because noise cannot be reduced.
Second, there is a problem in that a wide filtering curve provided by silicon amplifies any noise in a color correction step.
Third, Foveon's sensor has a fill factor lower than that of a CMOS APS sensor, and even that of an ILT CCD sensor.